A person or thing that puts someone or something in danger or at risk.
From endanger (to put in danger, from en- + danger) + -er (agent suffix). The verb 'endanger' comes from Old French dangier, meaning 'to have power over' or 'to jeopardize,' and the -er suffix makes it a person or thing that does the action.
The word 'endangerer' is rarely used because English speakers prefer saying 'person who endangers' rather than adding -er, showing how not all logical word combinations actually catch on in real speech.
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